(1) Field of the Invention
This invention relates to an apparatus for the continuous, homogeneous and effective mixing of polyol with low-boiling point foaming agent which are used as raw materials for the manufacture of a rigid foamed polyurethane. More particularly, this invention relates to an apparatus for the mixing of polyol with chlorodifluoromethane, CHClF.sub.2 (designated by HCFC-22), chlorodifluoroethane, CH.sub.3 CClF.sub.2 (designated by HCFC-142b), or the like which are used as low-boiling point foaming agents for the manufacture of the rigid foamed polyurethane of the invention. These low-boiling point foaming agents have a short life in the atmosphere because of the presence of hydrogen atoms in the molecules, and have, therefore, less suspicion of being substances that are eating a hole in the atmosphere's layer of ozone, which consequently are linked to global warming.
(2) Description of the Prior Art
It is known that rigid foamed polyurethanes made up substantially of closed cells, which are manufactured by use of a polyol, an isocyanate, water, trichlorofluoromethane, CCl.sub.3 F (designated by CFC-11; boiling point 23.8.degree. C.) as foaming agent, have good mechanical and thermal insulating properties, excellent dimensional stability, sound insulating and electrical properties, and chemical resistance. Accordingly, they are widely used in the making of thermal insulating materials for refrigerators, freezers, chill cars, iceboxes, containers for frozen food, LPG tanks; insulating materials for panels of curtain walls; building materials for roofing or canopies of cold storage warehouses; and various parts of automobiles.
Lowland of the University of California disclosed a theory in 1974 that specific CFCs which include trichlorofluoromethane (CFC-11), dichlorodifluoromethane (designated by CFC-12), trichlorotrifluoroethane (designated by CFC-113), dichlorotetrafluoroethane (designated by CFC-114) and chloropentafluoroethane (designated by CFC-115), are suspected of being ozone-destroying substances. The CFCs have been linked to the cause of an Antarctic hole in the Earth's ozone layer, which screens skin cancer-causing ultraviolet radiation from the sun.
Further, the CFCs are said to have a long-life, for example, for about 100 years in the environment and they absorb infrared light. Therefore, the CFCs are also believed to be so-called greenhouse gases; substances that may contribute to global warming that could cause the earth's temperature to rise with potentially catastrophic effects for health and production of crops.
Their use is strictly regulated in technological by advanced countries and those regulation are expected to be tightened still further in the future.
Their use and production have been regulated in Japan since 1989, and a complete ban on the CFCs production and use will be in effect in the near future. From this point of view, it is an urgent matter to develop substitutes for the CFCs.
CFC-11 which is included in the specific CFCs and used widely in making foamed plastics as a foaming agent, is similarly circumstanced. CFC-11 has a boiling point of 23.8.degree. C., and it liquefies in the closed cells of polyurethane when the polyurethane is used in making thermal insulating materials for a refrigerator, a freezer, or a show case which is maintained at about -30.degree. to -40.degree. C. In this case, the thermal conductivity of the insulating material changes sharply, yielding no effectual thermal insulating results.
Therefore, there is a need to develop a process for the manufacture of a rigid foamed polyurethane by the use of a foaming agent which does not liquefy under such cold conditions and makes the foamed polyurethane retain its low thermal conductivity.
For the mixing of a polyol and a relatively high-boiling point foaming agent such as CFC-11, as raw materials for the manufacture of a rigid foamed polyurethane, a batchwise mixing apparatus has been extensively used. However, it is not possible to convert the batchwise apparatus into a new one used for the mixing of a foaming agent, such as HCFC-22, which vaporizes at ordinary temperatures.
Therefore, there is a need for developing a process for the manufacture of a rigid foamed polyurethane by means of a mixing apparatus by which the polyol and the relatively low-boiling point foamed agent, such as HCFC-22, may be efficiently mixed.